Thermoplastic resins have excellent heat resistance, mechanical properties, impact resistance and dimensional stability and are widely used in fields such as an office automation equipment field, automobile field and electric/electronic part field. On the other hand, however, the thermoplastic resins also have an aspect that most of raw materials thereof rely on oil resources.
In recent years, in view of fear of exhaustion of the oil resources and a problem of increase of carbon dioxide in air that causes global warming, carbon-neutral biomass resources that do not rely on oils for raw materials and do not increase carbon dioxide by combustion thereof have been gathering great attention. Even in the field of polymers, biomass plastics produced from biomass resources have been ardently developed.
Representative examples of the biomass plastics include polylactic acids, and use thereof has been increasingly expanded to dishes, packaging materials, miscellaneous goods and the like, because they have relatively high heat resistance and mechanical properties among the biomass plastics. In addition, the polylactic acids have also been studied for a possibility as industrial materials.
However, the properties of the polylactic acids such as mechanical properties and heat resistance are unsatisfactory in order for the polylactic acids to be used as industrial materials in fields in which thermoplastic resins are used. Further, the polylactic acids have a problem that they show significantly low hydrolysis resistance when used under wet and hot conditions because they have biodegradability.
Further, the polylactic acids have optical isomers, and it is known that when a poly-L-lactic acid that is a polymer of L-lactic acid and a poly-D-lactic acid that is a polymer of D-lactic acid are mixed together, they form stereocomplex crystals which are a material showing a higher melting point than crystals of the poly-L-lactic acid alone or the poly-D-lactic acid alone (see, Patent Document 1, Nonpatent Document 1). An attempt to apply this stereocomplex polylactic acid to industrial applications such as automobile parts and household appliance parts by taking advantage of its heat resistance has been made (see, Patent document 2).
However, when this stereocomplex polylactic acid is produced by an industrially advantageous melt-extrusion process, it is very difficult to achieve stereo-complexification to a sufficient level. When stereo-complexification is insufficient, the stereocomplex polylactic acid cannot exhibit good heat resistance that is a characteristic thereof. Further, although the stereocomplex polylactic acid has a tendency of showing a higher crystallization rate than the poly-L-lactic acid or poly-D-lactic acid, it is still insufficient to produce the stereocomplex polylactic acid efficiently by injection molding. As described above, even the stereocomplexpolylactic acid still has a number of problems in order to be used in wide applications.
Under the circumstances, an attempt to replace some of plastics which rely on oils for raw materials by biomass plastics has recently been becoming popular as a measure for reducing environmental burdens of the plastics, and as to thermoplastic resins, it has been proposed to incorporate natural polymers such as corn starch into the resins (see, Patent document 3). Further, compositions comprising thermoplastic resins and polylactic acids have been proposed (see, Patent documents 4 to 6).
As for polylactic acids, attempts to use them as industrial materials by introducing resins such as aromatic polycarbonates and a flame retardant into the polylactic acids have been made (see, Patent document 7)
However, it is the current situation that a problem of decrease of hydrolysis resistance that is derived from a characteristic of the polylactic acids has not yet been solved and hinders the above materials from being used as industrial materials in various fields.    (Patent document 1) Japanese Patent Laid-Open Publication No. 63-241024    (Patent document 2) Japanese Patent No. 3583097    (Patent document 3) Japanese Patent Laid-Open Re-Publication No. 7-506863    (Patent document 4) Japanese Patent No. 3279768    (Patent document 5) Japanese Patent Laid-Open Publication No. 2005-48066    (Patent document 6) Japanese Patent Laid-Open Publication No. 2005-48067    (Patent document 7) Japanese Patent Laid-Open Publication No. 2004-190026    (Nonpatent document 1) Macromolecules, 24, 5651 (1991)